In recent years, organic electronic materials have been paid to attention. The organic electronic material refers to those which are intended to apply organic materials as a new material in place of semiconductors using the current inorganic materials. Among practical electronic devices using organic materials, so-called active devices using active functions generated by flowing the current to materials have extremely been limited. The most typical example thereof is an organic electrophotographic photoconductor. Some examples which have currently been under development include an organic electroluminescent device, an organic solar cell, an organic transistor and the like.
As an organic electronic material, there are a hole transporting material (organic p-type semiconductor) in which a hole acts as a charge carrier and an electron transporting material (organic n-type semiconductor) in which a electron acts as a charge carrier. Since material development up to now has been mainly carried out for hole transporting materials, a large number of hole transporting materials have been known. Electron conduction hardly takes place due to formation of deep trap level resulted from oxygen molecules having a large electron affinity in electron transport so that the number of electron transporting materials is extremely small as compared to that of hole transporting materials.
The organic electronic device is used in combination of a hole transporting material and an electron transporting material in many cases. There has been no high performance electron transporting material as yet, so there have been some problems in the organic electronic device. Various devices as described above will be explained in more detail below.
An organic electrophotographic photoconductor (OPC) has been widely put to practical use from the viewpoint that characteristics of a photoconductor can be designed in many ways because of pollution-free property, low cost and some degree of freedom of material selection. As for a photosensitive layer of OPC, there have been proposed a so-called function-separated type photoconductor such as dual-layered type photoconductor in which a charge generation layer and a charge transport layer are stacked, and a so-called single-layered photoconductor in which a charge generating material and a charge transporting material are dispersed in a single photosensitive layer, and the like.
Charge transporting materials used in these photoconductors are required to have high carrier mobility. However, since most charge transporting materials with high carrier mobility have been used for transporting holes, OPC actually provided for the practical use has been limited to a dual-layered type photoconductor in a negatively charged process with a charge transport layer provided on an outermost layer from the viewpoint of mechanical strength. However, OPC in a negatively charged process uses the negative corona discharge. So, there are some problems such that it is unstable as compared with that using the positive corona discharge, and ozone, nitrogen oxide or the like is generated and adhered on a surface of the photoconductor, easily causing physical and chemical deterioration, and exerting bad influence on the use environment.
In order to solve the above problems, OPC which can be used for positively charged process is effective. For this reason, an electron transporting material is required to be used as a charge transporting material. 2,4,7-Trinitrofluorenone is known as an electron transporting material. However, such a substance is not sufficient in solubility to and compatibility with a solvent or a binder polymer, and it does not have sufficient properties to actually constitute a photosensitive layer. Further, its use has been stopped because of its carcinogenicity as well.
In late years, for example, in Patent Document 1, there has been proposed to use a compound having a diphenoquinone structure or a benzoquinone structure as an electron transporting material for an electrophotographic photoconductor. Further, in Patent Document 2, there has been proposed to use a benzenetetracarboxylic acid diimide compound as an electron transporting material for an electrophotographic photoconductor.
However, since conventional electron transporting materials such as diphenoquinone derivatives, benzoquinone derivatives, benzenetetracarboxylic acid diimide compounds and the like have low compatibility with binder polymers, there are problems such as precipitation and the like. Further, the amount capable of dispersing in a photosensitive layer is limited, thus increasing the hopping distance. So, under low electric field, the movement of electrons hardly takes place. Therefore, it is difficult to make the conventional photoconductor comprising an electron transporting material into a photoconductor with excellent electron transporting capability.
A thin film transistor has been widely used as a switching device for liquid crystal display device or the like. In the past, a thin film transistor (TFT) has been prepared by using amorphous silicon or polycrystalline silicon. However, CVD apparatus to be used for the production of TFT using this silicon is highly expensive so that there is a problem in that the production of large-sized display devices using TFT or the like is accompanied by a sharp increase in the production cost. Further, the process for making amorphous silicon or polycrystalline silicon into a film is carried out at a very high temperature, thus the kind of materials which can be used as a substrate is limited. Therefore, there is a problem that and lightweight polymer substrate or the like cannot be used.
In order to solve the above problems, TFT using an organic semiconductor has been proposed instead of using amorphous silicon or polycrystalline silicon. A thin film transistor using an organic semiconductor has been actively developed little by little since late 1980s, and in late years, basic performance has exceeded characteristics of a thin film transistor made of amorphous silicon. As a method for forming a film used for forming TFT with an organic semiconductor, there have been known a vacuum deposition method, a coating method and the like. However, according to these film-forming methods, large-sized devices can be realized while holding down an increase in cost, and a process temperature required for film-forming can be relatively low. For this reason, TFT using an organic semiconductor (hereinafter referred to as “organic TFT”) is provided with an advantage in which limitation on the selection of to be used as a substrate is low, and it is expected to be put to practical use. Further, a TFT has also been paid attention to possibility of use as a smart card or a security tag.
Performance of an organic TFT mainly depends on charge mobility of an organic compound and an on/off ratio of current. Therefore, ideally, it needs to have low conductivity, along with high charge mobility, while the current is off. The on/off ratio herein refers to a ratio of the current between the source and drain when the organic TFT is on to the current between the source and drain when the organic TFT is off.
Organic substances are used in simple substance or in combination with other compounds for an organic compound layer of an organic TFT. The organic substances contain conjugated polymers, multiers of thiophene or the like, metal phthalocyanines or condensed aromatic hydrocarbons such as penthacene or the like. However, as described above, a study on an organic TFT has been actively carried out, whereas any of the conventional organic TFT was slow in its operational speed and could not have practically sufficient on/off ratio since performance of a compound in use was insufficient.
An organic electroluminescent device is excellent in impact resistance because it is a perfect solid device and its visibility is high because of its self-emission property. Therefore, at present, the organic electroluminescent device has been actively studied as a flat panel type display. This organic electroluminescent device has a structure of successively stacking a hole injecting electrode, an organic layer and an electron injecting electrode on a transparent glass substrate. As the hole injecting electrode, an electrode material having a high work function such as Au (gold) or ITO (indium tin oxide alloy) is used, while as the electron injecting electrode, an electrode material having a low work function such as Mg is used. Further, for the aforementioned hole injecting and transporting layer, an organic material having a property of a p-type semiconductor has been used, while for the electron injecting and transporting layer, an organic material having a property of an n-type semiconductor has been used.
The principle of light emission of the organic electroluminescent device is considered that excitons are generated by the recombination of holes injected from the hole injecting electrode and electrons from the electron injecting electrode at an interface between a light-emitting layer and a hole (or electron) transporting layer, and in the light-emitting layer, and the excitons serve to excite molecules of a light-emitting material constituting the light-emitting layer.
However, electroluminescent materials are classified into organic and inorganic electroluminescent materials. As an organic electrolumihescent material, single crystalline anthracene emitting blue light has been known from the past, while as an inorganic electroluminescent material, a compound semiconductor has been well known. However, an anthracene single crystal is thick, i.e., from several tens of μm to several mm. So, in order to emit light from this single crystal, a drive voltage of several hundreds of V was needed. Further, there has been a problem such that the electron injecting efficiency is low for injecting both charges of holes and electrons to this single crystal because the anthracene single crystal is an organic material of a single composition. Further, a drive voltage necessary for light emission of the crystal could be reduced by thinning this anthracene single crystal, but it was difficult to improve the electron injecting efficiency.
In Non-Patent Document 1 by Tang and VanSlyke of Kodak Company in 1987, there has been reported a device composed of two layers such as a hole transporting layer and an electron transporting light-emitting layer which emits green light with good efficiency at a lower drive voltage of approximately 10 V, as compared to the conventional organic electroluminescent device of a single-layered structure.
The reason why the light-emitting efficiency is improved due to this multi-layered structure as compared to the past is because a balance of holes and electrons injected from electrodes can be achieved. In the above devices, the hole transporting layer has the function of injecting holes from an anode to the electron transporting light-emitting layer and at the same time prevents electrons injected from a cathode from running away to the anode without the recombination with holes for playing a role of blocking electrons up in the electron transporting light-emitting layer. For this reason, by the effect of blocking electrons by this hole transporting layer, the recombination of holes and electrons takes place with much better efficiency, as compared to the conventional single layer devices, thus enabling a big reduction in the drive voltage.
Moreover, excitons generated by the recombination also have the function of preventing radiationless deactivation on a surface of a metal electrode. From this point of view, a hole blocking material has been under development. A hole blocking layer is located between the light-emitting layer and the electron injecting and transporting layer, and has the effect of blocking charges (holes or electrons) or excitons up in the light-emitting layer. The following compounds have been reported so far. Oxadiazole derivatives (Patent Document 3) have been widely used until now, but there is a problem such that crystallization easily takes place. Even when other compounds are used, there are some problems such as an increase in the drive voltage and the like. For this reason, in order to develop an organic electroluminescent device which has much higher light-emitting efficiency and longer lifetime, development of an electron injecting and transporting material has been demanded.
Solar energy has been actively studied for its use as an environment-friendly energy. An inorganic semiconductor such as silicon, CdS, CdTe, CdAs and the like has been widely used for a solar cell from the viewpoint of high solar energy conversion efficiency in the past. The conversion efficiency thereof reaches about 15%, when, for example, silicon is used. However, in the solar cell using the inorganic semiconductor, since many processes such as a process of producing a single crystal, a doping process and the like are required for the production of the cell, this causes a problem of greatly increasing the production cost.
In order to reduce the cost involved in the production of this solar cell, an organic solar cell using an organic semiconductor capable of easily producing a thin film by vapor deposition, casting or the like has been studied. A solar cell using an organic semiconductor has many advantages as compared to a solar cell using an inorganic semiconductor, but the conversion efficiency is low so that such a solar cell could not be put to practical use. For example, there has been reported a so-called Schottky barrier-type device using the contact between metal-free phthalocyanine and aluminum by Loutfy et al. (Non-Patent Document 2). However, when the intensity of irradiated light is increased, the conversion efficiency is suddenly decreased or the device becomes worsened over time. This is because aluminum in an electrode becomes oxidized by oxygen in air. There has been reported a solar cell in which a perylene derivative that is an organic n-type semiconductor is connected with phthalocyanine, in place of aluminum by Tang et al. (Non-Patent Document 3). This solar cell exhibits the conversion efficiency of 1% under an artificial sunlight, achieving the highest conversion efficiency for the present. This is because the spectrum sensitivity can be enlarged because of the solar cell capable of carrying out light irradiation from a transparent electrode and generating a photogenerated charge carrier with two kinds of materials. However, the conversion efficiency is still low as compared with that of a solar cell employing an inorganic semiconductor, so the efficiency is required to be improved about 10 times more for the practical use.
One reason of the low photoelectric conversion efficiency of the organic solar cell is a difference in carrier generating mechanisms of an inorganic semiconductor and an organic semiconductor. The interlattice interaction in the inorganic semiconductor is strong so that an electron-hole pair is created directly by the light absorption. On the other hand, since the intermolecular interaction in the organic semiconductor is weak, i.e., just about 0.1 eV, the energy perturbation due to lattice defects or impurities is small, strongly bound excitons of the Frenkel type are generated by the light absorption, and usually free carriers are not directly generated.
The second reason is that an active area for the generation of optical carriers in the organic semiconductor is narrow. In the conventional simple p-n junction type organic solar cell, the width of an area activated for the generation of carriers to be formed near the junction is very narrow, and an organic semiconductor layer other than an activated area near the junction (usually about several tens of nm) becomes a dead layer which does not generate carriers even when light is absorbed.
That is, the operational principle of the organic solar cell is that the excitons generated by light absorption reaches the activated area near the junction while diffusing, and generates free carriers.
A three layer device composed of a mixed layer of phthalocyanine of an organic p-type semiconductor and a perylene derivative of an organic n-type semiconductor inserted between a p-type layer and an n-type layer of the p-n junction type organic solar cell has been reported by Yokoyama et al. (Non-Patent Document 4). The three layer device having a mixed layer exhibits light current value of two times or more than the two layer device without having the mixed layer. However, it is not possible to effectively transport light carriers generated from the mixed layer to an electrode, so much improvement of the efficiency has been required for the practical use.    [Patent Document 1] Japanese Patent Laid-Open No. 1989-206349    [Patent Document 2] Japanese Patent Laid-Open No. 1993-142812    [Patent Document 3] Japanese Patent Laid-Open No. 1990-216791    [Non-Patent Document 1] Appl. Phys. Lett., Vol. 51, No. 12 (1987), pp. 913-915    [Non-Patent Document 2] J. Chem. Phys., Vol. 71, p. 1211    [Non-Patent Document 3] Appl. Phys. Lett., Vol. 45, p. 1144    [Non-Patent Document 4] J. Appl. Phys., Vol. 72, p. 3781